1. Field of the Invention
The present invention relates to a solid electrolyte battery having excellent heavy load resistance, pulse discharge load resistance and over discharge resistance.
2. Description of the Related Art
Since a variety of electronic apparatuses have dramatically progressed, research and development of secondary battery have been carried out. The reason for this lies in that the secondary battery can continuously be used for a long time and the same can again be charged to serve as an economical power source. The representative secondary batteries are exemplified by a non-aqueous electrolyte secondary battery, such as a lithium ion battery, and an all-solid electrolyte polymer battery. The all-solid electrolyte polymer battery incorporates all-solid polymer electrolyte containing a cation conductive materials, such as polyether oxide, polyester, polyimide, crosslinked polyether or polyether derivatives.
In particularly, the all-solid electrolyte polymer battery incorporates a heating-permissible electrolyte. Since the foregoing electrolyte is embedded in high polymer chains of the polymers, frame resistance and a self-extinguishing characteristic can be imparted. As compared with the non-aqueous electrolyte secondary battery, the all-solid electrolyte polymer battery has significant safety characteristics to resist disturbance, such as an external shock, unintentional overcharge and unintentional leaving in a hot atmosphere. As described above, the all-solid electrolyte polymer battery has satisfactory safety.
The all-solid electrolyte polymer battery suffers from a problem in that a poor ion conductivity .sigma. of 10.sup.-4 S (siemens)/cm to 10.sup.-5 S/cm in the vicinity of room temperature. As described above, the polymer battery incorporates the all-solid electrolyte polymer battery having superior safety to that of the non-aqueous electrolyte secondary battery. Moreover, the polymer battery contains the electrolyte which is the non-aqueous electrolyte having the poor ion conductivity embedded in the polymers. Therefore, there arises a problem in that an output of an electric current which can be obtained per unit hour is excessively small as compared with that of the non-aqueous electrolyte secondary battery. As a result, there arises a problem in that use of the all-solid electrolyte polymer battery is limited.
Therefore, a polymer battery having an ion conductivity of 10.sup.-3 S/cm or greater which is a practical value from the viewpoint of industrial use has been obtained and investigated. Note that all-solid electrolytes have been reported which exhibit a high ion conductivity of 10.sup.-3 S/cm at high temperatures in the vicinity of 100.degree. C. which is a level required from the viewpoint of practical use. However, the above-mentioned high temperature condition is not a condition under which the battery is frequently used from a viewpoint of the practical use. Therefore, simplicity and the shape of the polymer battery are limited in the field of use of the polymer battery.
An attempt to improve the ion conductivity of the all-solid electrolyte polymer battery has been made by raising the concentration of salt in the polymer electrolyte. However, the above-mentioned method cannot improve the ion conductivity. The reason for this lies in that rise in the concentration of salt in the polymer electrolyte enhances crystallization of the polymers. As a result, the ratio of ions which can freely be moved in the polymer electrolyte cannot be raised.
Another method has been investigated in which the concentration of salt in the polymer electrolyte is raised and a crystallization inhibitor, such as isocyanate, is mixed with the polymer. The above-mentioned method, however, causes isocyanate to react with cation. What is worse, introduction of active protons results in the severer conditions being required. Therefore, the foregoing method is not an effective method.
Another method has been investigated which improves the ion conductivity of the all-solid electrolyte polymer battery. The method uses molten salt to raise the density of carriers. Although lithium double salt has solubility, the double salt can easily be exploded. Therefore, safety cannot easily be realized.
Another means has been disclosed in which side-chain molecular structures provided for main chains of the polymers have the ion conductivity in place of the main chains of the molecular structures arranged in the polymers to form a mesh configuration. If the foregoing method is employed, a required ion conductivity cannot be obtained. However, a poor ion conductivity at room temperature is realized which is about 10.sup.-4 S/cm which is lower than a practical value by one digit.
As a practical means for achieving an ion conductivity .sigma. of 10.sup.-3 S/cm, a composite electrolyte has been disclosed which has the ion conductivity by impregnating a porous film made of polypropylene or polyvinylidene fluoride with organic solvent, such as polycarbonate, ethylene carbonate or .gamma.-butyllactone. As a matter of course, the foregoing porous film, however, has unsatisfactory mechanical strength. As a result, short circuit easily occurs when external force is exerted. In particular, the polymer electrolyte cannot resist dendrite short circuit, causing the organic solvent with which the porous film has been impregnated to easily be exudated. Therefore, a battery manufactured by the above-mentioned method cannot be included in the category of the polymer electrolyte battery.
As another composite electrolyte, a method has been disclosed which uses a high polymer gel film in which non-proton organic solvent is confined in small spaces in a 3D network structure of crosslinked high polymer molecules.
The electrolyte containing the high polymer gel film has required mechanical strength as compared with that of the all-solid electrolyte. Moreover, an ion conductivity of an order of 10.sup.-3 S/cm can be obtained. Therefore, the foregoing method is considered to be a most practical solving means.
As compared with the conventional non-aqueous electrolyte secondary battery, such as the lithium ion battery, the polymer battery incorporating the high polymer gel film does not require a pressure-resisting container because organic solvent is confined in the gel form polymer. Moreover, a weight energy density and a volume energy density can be obtained which are realized when the foregoing battery has been formed into a battery pack and which are similar to or superior to those of the conventional non-aqueous electrolyte secondary battery. The polymer battery incorporating, the high polymer gel film is able to maintain the shape thereof with thin structures of electrodes. Therefore, the foregoing polymer battery has a possibility that a very thin battery cell can be manufactured. Therefore, there has been expected to apply the foregoing polymer batteries to notebook personal computers and very-thin portable apparatuses. Similar to the all-solid electrolyte polymer battery, the polymer battery incorporating the high polymer gel film contains the high polymer chains of the polymers which are impregnated with the electrolyte. Therefore, frame resistance and a self-extinguishing characteristic can be imparted, causing, satisfactory safety to be realized.
As described above, the polymer battery incorporating, the high polymer gel film has the ion conductivity of the electrolyte, which is 10.sup.-3 S/cm, similar to that of the electrolyte of the non-aqueous electrolyte secondary battery. However, the foregoing polymer battery suffers from low reaction speed in the interface between the electrolyte and the active materials as compared with the non-aqueous electrolyte secondary battery. Therefore, an output of electric currents which can be obtained per unit hour is inferior to that of the non-aqueous electrolyte secondary battery.
As compared with other secondary batteries including a Ni--Cd secondary battery and a Ni--MH secondary battery, the all-solid electrolyte polymer battery and the polymer battery incorporating the high polymer gel film have poor discharging characteristic when a large current is discharged. In actual, the polymer battery incorporating the high polymer gel film has a double electric layer having a relatively large capacitance and formed in the interface between the active materials and the electrolyte owning to polarization of the electrolyte. Although electric charges accumulated in the double electric layer are first discharged when instantaneous discharge in a certain quantity is required. The amount of the electric charges is smaller than a required amount.
When the all-solid electrolyte polymer battery or the polymer battery incorporating the high polymer gel film is employed as a power source for an automobile or a power surface for another industrial field, there arises the following problem. That is, each of the foregoing batteries cannot satisfactorily be applied to a case in which discharge of a large electric current is instantaneously required when the operation of the apparatus is started or the operation speed is accelerated.
Specifically, when the all-solid electrolyte polymer battery or the polymer battery incorporating the high polymer gel film is employed as the power source for an automobile, a large electric current, which is several times the electric currents required to perform a usual operation, is required to start or accelerate the automobile. When the polymer battery is employed as the power source for an automobile, a period of time required to start or accelerate the automobile is a short time with respect to the overall period of time for which the automobile is driven. A considerably large electric current is required in the foregoing short period of time. Therefore, a battery cell must be provided which has a capacitance which is several times a capacitance required to perform a usual operation. As an alternative to this, a capacitor having a large capacitance must be provided in addition to the battery Moreover, an operation for performing control to switch the power source on the circuit must be performed to supply the large electric current in the short period of time in which the start or the acceleration is performed.
A state will now be considered in which the all-solid electrolyte polymer battery or the polymer battery incorporating the high polymer gel film is employed as the power source for a portable telephone. In the foregoing case, a satisfactory load characteristic which is able to resist discharge of large electric currents of several meter seconds to some hundreds meter seconds, that is, so-called pulse discharge, is required. However, a polymer battery having a satisfactory load characteristic to resist the pulse discharge has not been realized as yet.
The solid electrolyte polymer battery or the polymer battery incorporating the high polymer gel film has the ion conductivity of the electrolyte which easily depends on the temperature. Therefore, there arises a problem in that the ion conductivity deteriorates at low temperatures and thus the load characteristic deteriorates. That is, the electrolyte of the polymer battery suffers from a problem in that the ion conductivity excessively depends on the temperature.
When the temperature has been lowered, the ion conductivity of the all-solid electrolyte polymer battery excessively deteriorates as compared with that indicated with the Arrhenius equation of a solution-type electrolyte. In particular, the ion conductivity rapidly deteriorates at temperatures lower than temperatures in the vicinity of point Tg which is a glass transition point. For example, ion conductivity .sigma. of polyether oxide at 0.degree. C. or lower is 10.sup.-6 S/cm to 10.sup.-8 S/cm which are similar to those of an insulating material.
When the foregoing battery is mounted on an automobile, an industrial apparatus or a portable telephone, deterioration in the load characteristic at low temperatures, for example, -20.degree. C. or lower must be overcome to practically use the battery.